1. Field of the Invention
The present invention relates to a vertical type processing apparatus which is used to apply etching and ashing processes to matters such as semiconductor wafers.
2. Description of the Related Art
Various kinds of processes such as film forming, etching and ashing processes are usually included in the course of manufacturing semiconductor devices. Recently, in accordance with demand of increasing the density of these devices and making them more highly integrated, the use of plasma has been tried these days in these processes.
Plasma contains positive ions and electrons, and is kept neutral as a whole. Also present in plasma are activated radicals. These activated radicals are constituted by exited atoms and particles which are generated when the positive ions and electrons collide with neutral particles. Ions and activated radicals contribute to the film forming, etching and ashing processes.
Published Unexamined Japanese Patent Application No. 58-204526, for example, discloses an apparatus which uses plasma to process a plurality of wafers at once. According to this processing apparatus, the reaction tube is set horizontal and plasma process is applied to the semiconductor wafers housed vertical in the reaction tube.
In this processing apparatus, however, plasma is generated along the inner wall of the reaction tube. Plasma cannot be therefore uniformly applied to the surface of each semiconductor wafers which is located on the bottom portion of the reaction tube. Namely, a larger amount of plasma is supplied to that portion of the surface of each semiconductor wafer which is nearer to the bottom of the reaction tube. Thus, surface of each semiconductor wafer cannot be uniformly processed. Further, since the semiconductor wafers are exposed directly to plasma in this processing apparatus, ion sheath is thus formed on the portion adjacent to the surface of each semiconductor wafer, and they are sputtered by ions in plasma. The semiconductor wafers are thus damaged and the yield of the semiconductor wafers lowers.
Published unexamined Japanese Utility Model Application No. 2-11327, for example, discloses a plasma processing apparatus of the vertical type. The reaction tube is erected in this case and the wafer boat in which plurality of semiconductor wafers are horizontally stacked is loaded into the reaction tube. The plasma process is then applied to the wafers. This plasma processing apparatus includes plasma generating electrodes arranged in the reaction tube so as to enclose the wafer boat.
According to this plasma processing apparatus, each of the semiconductor wafers is positioned in the reactor tube to have an equal interval from its outer circumferential rim to the electrodes. The surface of each semiconductor wafer can be thus uniformly processed. Namely, uniform plasma process can be applied to the surface of each semiconductor wafer.
In the vertical type plasma processing apparatus, however, the plasma generating electrodes are arranged in the reaction tube. Ions in plasma, therefore, sputter the electrodes, thereby causing electrode components to enter, as impurities, into the semiconductor wafers. Even a very small amount of these impurities included in the semiconductor wafers lowers the yield of the semiconductor wafers.